Conventional liquid crystal display elements include a glass substrate typically having dimensions of about 300 mm.times.400 mm.times.0.7 mm. Recent liquid crystal display elements include a larger glass substrate of a 400 mm or 500 mm size.
In a manufacturing process of such liquid crystal display elements, the glass substrate is moved past, for example, a resist applying apparatus, an exposure apparatus, an orientation film printing apparatus, a spacer dispersion apparatus, a seal printing apparatus, and the like. During operation by the apparatus, the glass substrate 51 is fixed onto a flat upper surface of a support table (hereinafter will be referred to as a stage) 52 by vacuum attraction as shown in FIGS. 6 and 7.
Specifically, the stage 52 has attraction openings 53 having a diameter of about 0.5 mm to 1.0 mm arrayed in a matrix form with intervals of about 5 mm to 30 mm. The attraction openings 53 are connected via a linking passage 54 to a vacuum pump 55 disposed underneath the stage 52. The glass substrate 51 is attracted by vacuum attractive forces exerted simultaneously across nearly the entire glass substrate 51 at the attraction openings 53, and thus fixed onto the stage 52.
The glass substrate 51, made to be like a flat panel with superb flatness and rigidity, dose not warp and is fixed onto the stage 52 uniformly even if attractive forces are exerted in a discrete manner thereonto at the attraction openings 53 simultaneously as explained above. As a result, for example, in the orientation film printing treatment of the manufacturing process of a liquid crystal display element, an orientation film is uniformly applied with no printing irregularities such as convexities and concavities in the finished state.
Moreover, as disclosed in Japanese Laid-Open Patent Application No. 9-80404/1997 (Tokukaihei 9-80404), a slightly bent glass substrate 51 having a thickness of about 0.7 mm can be attracted without vacuum breakdown by vacuuming several regions with time shifts using a vacuum pump and a switching valve because of the weight of the glass substrate 51 itself.
The glass substrate 51 is disposed on the stage 52 by being positioned properly in a cassette loaded with a plurality of glass substrates 51 by means of positioning pins and the like and then placed on lift pins standing on the stage 52 by a transport arm and the like.
The glass substrate 51 is then placed on the stage 52 by lowering the lift pins and fixed onto the stage 52 by exerting vacuum attractive forces at the attraction openings 53 either simultaneously or with time shifts using the switching valve and the like as described above.
Thereafter, for example, in an orientation film printing apparatus and a seal printing apparatus, highly precise positioning is conducted using a CCD camera, alignment marks formed in advance on the glass substrate 51, etc. before proceeding to a further process.
A different attraction method, using a blower, is disclosed in Japanese Laid-Open Patent Applications No. 8-324786/1996 (Tokukaihei 8-324786) and No. 7-33281/1995 (Tokukaihei 7-33281).
In addition, thin-type glass substrates and thin-type plastic substrates made of plastic having a thickness of 0.7 mm or less are employed in recent development for thinner and lighter liquid crystal display elements, and some of them are already available for commercial use. The aforementioned attracting method is used for those thin-type glass substrates and plastic substrates.
However, there are problems with the attracting methods for thin insulating substrates such as the glass substrates and plastic substrates above.
When the insulating substrate is fixed onto a stage by exerting vacuum attractive forces at attraction openings either simultaneously or with time shifts as disclosed in Japanese Laid-Open Patent Application No. 980404/1997 after lowering the lift pins and thus placing the insulating substrate on the stage, air is sucked between the insulating substrate and the stage through the attraction openings. The air flow generates static electricity and may cause the insulating substrate to slide on the stage as much as 1 cm.
Especially, the thin-type glass substrate and plastic substrate having a thickness of 0.7 mm or less, being lighter than glass substrates having a thickness exceeding 0.7 mm, exert less pressure on the stage and are more likely to be displaced.
Such movement of the insulating substrate may push the alignment marks out of the visible area for the CCD camera and the like, causing the CCD camera and the like to fail to recognize the alignment marks and to conduct highly precise positioning.
Moreover, if the thin-type glass substrate or plastic substrate is heated to remain at a temperature higher than room temperature while undergoing treatments in apparatuses or transported from one apparatus to another, irregular temperatures inside the apparatuses cause the substrate to have non-uniform temperature. The substrate consequently may warp, undulate, or bend entirely or partially.
Depending upon the treatment, the deformation may become of a perpetual nature or disappear after the treatment, which renders the substrate back into the flat shape. Sometimes, the substrate is deformed in various manners during a treatment. The extent, direction, and location of such deformation also may be perpetuated, disappear after the treatment, and vary constantly during the treatment. Even if the temperature inside the apparatus is consistent, the deformations happen while raising or lowering the temperature of the substrate before or after the treatment and conducting a series of treatments at different temperatures.
If that deformation happens, when the thin-type glass substrate or plastic substrate is fixed onto a stage by exerting vacuum attractive forces at attraction openings either simultaneously or with time shifts after lowering the lift pins and thus placing the substrate on the stage, the substrate cannot be attracted at the first instance. Otherwise, although being attracted at the first instance, the substrate may come off later as the attractive forces yield to the deformation of the substrate and cause the vacuum to break down.
Especially the thin-type glass substrate and plastic substrate are less rigid than glass substrates having the same size but a thickness exceeding 0.7 mm, the rigidity being less likely to overcome the deformation. Therefore, the thin-type glass substrate and plastic substrate are easier to deform and more difficult to attract.
In addition, the thin-type glass substrate and plastic substrate are lighter than glass substrates having the same size but a thickness exceeding 0.7 mm. If the warp exceeds a certain level (1 mm to 2 mm in a convex shape or in four directions for a substrate of about 300 mm.times.400 mm), the weight of the substrate cannot overcome the warp and render the substrate back into the flat shape, and the substrate therefore cannot be attracted onto the stage.
If the substrate is not attracted onto the stage, post-treatments such as alignment cannot be conducted.
Meanwhile, plastic substrates having a thickness of 0.4 mm or less and a plastic film having a thickness of 0.3 mm or less are even less rigid and susceptible to deformation and undulation even without being heated. This is a self-bending phenomena or so-called droop: when a part of the substrate is supported, the remaining part of the substrate, not supported, bends because the rigidity thereof yields to the weight thereof.
If the orientation state of the molecules of the material constituting the substrate or a physical or chemical property of the substrate in cross-sectional directions is not isotropic, the substrate undulates. This phenomena is called curling.
Depending upon the treatment, such as how the substrate is supported, the deformation may become of a perpetual nature or disappear after the treatment, which renders the substrate back into the flat shape. Sometimes, the substrate is deformed in various manners during a treatment.
If that deformation happens, when the plastic substrate is fixed onto a stage by exerting vacuum attractive forces at attraction openings either simultaneously or with time shifts after lowering the lift pins and thus placing the substrate on the stage, the substrate cannot be attracted at the first instance. Otherwise, although being attracted at the first instance, the substrate may come off later as the attractive forces yield to the deformation of the substrate and cause the vacuum to break down, and post-treatments such as alignment cannot be conducted.
If attractive force is to be exerted on the substrate susceptible to deformation, the attracting method using a blower is effective as disclosed in Japanese Laid-Open Patent Applications No. 8-324786/1996 and No. 7-33281/1995.
A blower is for generating an air flow by rotating blades. Air goes in one side and out the other. An object placed on the in-side is attracted to the blower: an electric cleaner is a good example.
The blower does not boast absolute attractive force as strong as a vacuum pump. However, the blower is versatile to deformations of an object, since the blower is capable of attracting the object even when the attraction opening is not completely closed by the object and thus free from vacuum breakdown.
However, if attractive forces are uniformly exerted across an entire deformed thin-type glass substrate or plastic substrate as in the arrangements disclosed in Japanese Laid-Open Patent Applications No. 8-324786/1996 and No. 7-33281/1995, the substrate may successfully be attracted onto the stage only in a deformed shape. Consequently, for example, a uniform orientation film cannot be applied in the orientation film printing treatment due to the deformation of the substrate, resulting in printing irregularities such as convexities and concavities in the finished state.
Moreover, in a rubbing treatment as an example, the rubbing cloth rubs the distorted part of the substrate excessively, leaving scratches on the orientation film or causing the substrate to split.
If a blower is used to exert attractive forces simultaneously across an entire thin plastic substrate having a thickness of 0.4 mm or less or across an entire plastic film having a thickness of 0.3 mm or less, the blower produces a similar result: the blower can successfully attract the substrate onto the stage only in a deformed shape. Consequently, the substrate may irrevocably be folded or split due to print pressure or rubbing pressure exerted on the substrate, for example, during orientation film printing and rubbing treatments.